Method and apparatus for eliminating phase distortion in seismic signals



F. G. METHOD AND APPARATUS F OR ELIMINATING PHASE Dec. 9, 1958 BLAKEDISTORTION IN SEISMIC SIGNALS Filed June 16, 1954 INVENTOR FRANCIS a.BLAKE United METHOD AND APPARATUS FOR ELIMINATING PHASE DISTORTION INSEISMIC SIGNALS Francis G. Blake, Fullerton, Calif., assignor toCalifornia Research Corporation, San Francisco, Calif., a corporation ofDelaware Application June 16, 1954, Serial No. 437,264

1 Claim. (Cl. 340-15) This invention relates in general to methods andapparatus for use in seismic prospecting, and relates more specificallyto methods and apparatus for reducing or eliminating phase distortion inseismic detector signals.

In the art of seismic prospecting, seismic waves artificially generatedin the earth are utilized to actuate a plurality of seismic detectorswhich are responsive to reflections and refractions of such waves fromsubsurface strata and which produce an electrical output signal varyingin sympathy with the received seismic waves. The usual seismic detectorsignal contains a fairly Wide range of different frequency components,and the signal is usually filtered to some extent and then recorded. Inthe case of reproducible recording of seismic detector signals, thesignal is filtered prior to being recorded and then additional filteringoperations, usually in the laboratory, may be performed on thereproduced signal.

The seismic waves produced by the explosion or other source of seismicenergy are composed of a number of different frequency components, andowing to the trans mission characteristics of the earth, these differentfrequency components are differently altered in phase in passing throughthe earth. Additionally, after the seismic waves have been convertedinto an electrical signal by the seismic detector, the differentfrequency components of the seismic detector signal are differentlyaltered in phase by the elements of the seismic recording system, suchas filters, geophones and amplifiers. Such phase distortion, either inthe seismic waves or in the seismic detector signal, is undesirable,particularly if different detector signals are to be mixed or addedtogether to produce a composite signal, and such distortion usuallynecessitates some type of correction.

Broadly, the present invention contemplates methods and apparatus foranalyzing a seismic detector signal in a manner to substantially removethe phase distortion introduced either into the seismic waves intraveling through the earth or into the seismic detector signal by thevarious components of the recording and/or analyzing system.

In accordance with the present invention, the seismic detector signalhaving some type of phase distortion therein is reproducibly recorded,and the recorded signal is then reproduced in the reverse order fromwhich the signal occurred; that is, the last part of the originalrecorded signal is reproduced first. The reproduced, reversed signal isthen passed through a device or system having substantially the samecomplex transfer function as the system which introduced the phasedistortion into the signal.

When the phase distortion is the result of distortion of the differentfrequency components of the seismic wave by the earth, the devicethrough which the reproduced, reversed seismic detector signal is passedmay be an electrical network having an electrical complex transferfunction which is analogous to the acoustical complex transfer functionof the portion of the earth through which the seismic waves passed. Whenthe 2,864,072 Patented Dec. 9, 1958 phase distortion is produced by theelements of the seismic amplifying and recording system, the networkthrough which the reversed signal is passed may be a substantialduplicate of the first network, or it may, through suitable reversingconnections, be the first network itself.

In this reversed passage of the signal through the device or system, theseismic detector signal is distorted in phase, but this phase distortionhas the opposite sign of the original phase distortion, so that the netphase distortion is zero for the two passages of the signal through thedevice or system. This operation may be performed for any number ofnetworks or devices which introduce phase distortion into a seismicsignal.

The method and apparatus of this invention have particular applicationin correcting for the phase distortion introduced into the seismicdetector signal by electrical filtering networks. By passing thereversed seismic detector signal through a filter having substantiallythe same electrical characteristics as the filter which produced theoriginal distortion, the total effective phase shift is reduced tosubstantially zero and the total transmission is the square of thetransmission for one passage through the filter. Thus, by repeatedlypassing a signal an even number of times through a filter, and reversingthe sequence of the signal each time, the slope of the sides of theover-all filter frequency response curve may be as steep as desiredwithout introducing any substantial phase distortion in the final outputsignal.

It is therefore an object of the present invention to provide improvedmethods and apparatus for recording and analyzing seismic detectorsignals.

It is an additional object of the present invention to provide a methodfor analyzing a seismic detector signal which has been phase-distortedby pas age through an electrical network, in which method the signal isreversed in time and passed through an electrical network havingsubstantially the same electrical characteristics as the firstmentionednetwork to substantially remove the phase distortion.

It is a further object of this invention to provide a method foranalyzing an electrical seismic detector signal which has phasedistortion therein among the different frequency components as a resultof a system having a complex transfer function, in which method theelectrical signal is reversed in time and passed through an electricalnetwork having a complex transfer function analogous to the complextransfer function of the system which introduced the phase distortion,to substantially remove the phase distortion.

It is a further object of this invention to provide methods andapparatus for analyzing a seismic detector signal which has beenphase-distorted by passage through an electrical network, in whichmethod and apparatus the signal is reproducibly recorded, reproduced inthe reverse time sequence from which the signal occurred, and passedthrough an electrical network having substantially the same electricalcharacteristics as the first-mentioned network to substantiallyeliminate the phase distortion from the signal.

It is an additional object of the present invention to provide methodsand apparatus for filtering a seismic detector signal in which thesignal is passed through an electrical filter network an even number oftimes, with the signal reversed in time sequence on alternate passagesthrough the filter network, to filter the signal without introducing anysubstantial resultant phase distortion therein.

Objects and advantages other than those outlined above will readily beapparent from the following description when read in connection with theaccompanying drawing, in which:

Fig. 1 diagrammatically illustrates one embodiment of the inventionutilized in connection with the analysis of seismic signals, and

Fig. 2 diagrammatically illustrates an alternate embodiment of theinvention utilizedin connection with a plu rality of electrical networkswhich introduce phase distortion into a seismicv signal.

Referring to Fig. l, numeral 11 designates a seismic detector responsiveto seismic waves for producing an electrical signal varying in sympathywith the received seismic waves. The output from detector 11 is suppliedto suitable amplifying means 12, where the signal is amplified andsupplied to a filtering network 13. Filter 13 may have any desiredcharacteristics, but preferably this filter attenuates only a smallportion or band of the frequencies present in the original seismicsignal to preserve a maximum of the original components for recording.

The signal from filter 13 is recorded on a suitable reproduciblerecording medium such as a magnetic tape recorder 16 having a length ofmagnetizable tape 16:; and provided with at least one recording head 16band at least one pickup or reproducing head 16c. As is well known in theart, variations in the signals supplied to recording head 16b produce avariable magnetization of tape 16a as the tape passes under thisrecording head. The recorded information may be subsequently withdrawnfrom the tape a number of times through reproducing head 16c. Recorder16 is of the type in which tape 16a may be moved past reproducing head16c in either direction, either by reversing the drive connections ofthe recorder or by removing and reversing the length oftape 16a.

The output from reproducing head 160 is supplied to a device or networkwhich may be a network simulating the complex transfer functions of bothfilter 13 and the portion of the earth through which the seismic wavespassed in reaching the detector. That is, the network could have aportion representing an electrical analog of the complex acoustictransfer function of the portion of the earth through which the seismicwaves passed and a portion substantially identical to filter 13.Alternatively, the output from reproducing head 160 may be supplied to asecond filtering network 13, which has substantially the same electricaltransfer function as filter 13. After passing through filter 13', thereproduced signal is supplied to recording head 21b of anotherreproducible recording medium, such as a magnetic drum 21 having; arotor covered with a layer of magnetizable material 231a and providedwith at least one reproducing head 21c. Drum 21 may be driven by anysuitable means, such as a motor 22 capable of providing rotation of drum21 in either direction. As with recorder 16, variations in the signalsupplied to recording head 21b variably magnetize the portion of layer21a passing under this head as the drum rotates, and reproducing head210 is used to subsequently withdraw the recorded signal.

The output from reproducing head 210 is supplied to means showndiagrammatically at 25 for performing one or more difierent operationson the seismic signal. Such operations may be classified as signalprocessing operations and may include additional filtering or the mixingof signals from different seismic detectors in the spread to produce acomposite signal. The output from signal processing means 25 ispreferably supplied to recording means 26 and/or display means 27 forproducing both a permanent record of the signal and an oscilloscopicdisplay for rapid visual evaluation 'of the effects of the differentoperations performed on the signal.

In the operation of the system shown in Fig. 1, the signal generated byseismic detector 11 is amplified by amplifier 12 and filtered in device13 and recorded on reproducible recording means 16, as is well known inthe art. Filtering device 13 introduces some phase distortion among thedifferent frequency components of the seismic detector signal passingtherethrough so that the signal recorded on recorder 16 contains thephase distortion introduced into the signal by filter 13 and, usually toa lesser extent, by amplifier 12 and geophone 11. To substantiallyremove this phase distortion from the signal, the direction of rotationof tape 16a is reversed with respect to its directionof rotation duringrecording of the signal. As mentioned above, such reversal may beobtained by either providing recorder 16 with a reversible drivemechanism or by removing and reversing the tape 1611 on the tape drivemechanism. The recorded signal may then be reproduced in the reverseorder from that in which it was recorded, with the end of the originalsignal passing first under reproducing head 160. This reproduced,reversed signal is then supplied from reproducing head to filter 13'which has substantially the same electrical characteristics as filter13. Alternatively, the reproduced, reversed signal from reproducing head16c may be supplied to filter 13 through suitable switching means toinsure an exact duplication of the electrical characteristics of thefilter through which the original seismic detector signal passed.

In the embodiment shown in Fig. l, fi ter 13', being a substantialduplicate of filter 13,attenuates the signal from reproducing head 16cto substantially the same extent that filter 13 did; filter 13' alsointroduces the same degree or" phase distortion into the signal as wasintroduced by filter 13. However, since the signal passing throughfilter 13 is the reverse of the signal which passed through filter 13,and therefore the phase distortion introduced by filter 13' has theopposite sign of the phase distortion introduced by filter 13, the netphase distortion is zero for the two passages of the signal through thefilters 13 and 13. The total transmission of the signal for the passagesof the signal through filters 13 and 13' is the square ofthetransmission for the passage of the signal through one of the filters,so that, effectively, the signal has been more sharply filtered, and thephase distortion introduced by filter 13 has been substantially removed.

The reversed, substantial]y-undistorted signal. from filter 13 issupplied to recording head 21b of recorder 21 to record the signal onmagnetiza'ble layer 210. After recording, the direction of rotation ofthe drum is reversed and the recorded signal reproduced by head 210 inthe normal or original sequence in which the signal occurred. The signalfrom reproducing head 210 is then supplied to signal processing means25, where additional operations may be performed on the signal prior tore cording and/ or displaying on devices 26 and 27.

Fig. 2 illustrates an alternate embodiment of the invention adapted tocompensate for the phase distortion introduced into the seismic detectorsignal by a plurality of electrical networks or devices in the seismicrecording system. Such distortion-producing device may be, as shown inFig. 2, the filtering network used in the field to filter the originalseismic signal prior to recording and the filtering networkssubsequently utilized, usually in the laboratory, for additionalfiltering of the signal reproduced from the original record. In Fig. 2,reference numerals 11, 12, 1'3 and 16 again designate, respectively,the. seismic detector responsive to the seismic waves for producing thesignal, the amplifier for amplifying the sig nal from detector 11, thefiltering device 13 for filtering the signal prior to the originalrecording, and the record ing means for reproducibly recording thesignal.

Reproducing head 16c of recorder 16 is again connected to a filteringdevice 13' having substantially the same electrical characteristics asfiltering device 13. The signal from filter 13 is supplied to afiltering device 31 representing a network for producing furtherfiltering of the seismic detector signal. The signal from filter 31 issupplied to recordinghead 21bof magnetic drum 21 for recording thesignal on the drum. The signal reproduced by head 21c is supplied to afilter 31 having substantially "the "same electrical characteristics asfilter 31. The signal from filter 31 is supplied to signal processingmeans 25, and the output thereof is impressed on recording means 26and/or display means 27.

To the operation of the embodiment illustrated in Fig. 2, the signalgenerated by seismic detector 11 is amplitied in amplifier 12 andsupplied to filter 13, where the Signal is filtered to the desiredextent and supplied to recording head 16b of recorder 16. As before,filter 13 introduce some phase distortion in the signal transmittedtherethrough. After recording the signal on re corder 16, the directionof rotation of tape 16;: is reversed and the signal reproduced throughreproducing head 16c in the reverse order from that in which itoccurred. The reversed signal is supplied to filter 13, where the phasedistortion introduced by filter 13 is substantially removed, asdescribed above in connection with Fig. 1, and the signal additionallyfiltered.

After passage through filter 13 the signal is supplied to filter 31,where additional filtering is performed. Filter 31 also introduces sometype of phase distortion into the signal transmitted therethrough sothat the output of filter 31, which is recorded on recorder 21, containssome phase distortion. After recording on recorder 21, the direction ofrotation of the drum is reversed and the signal recorded thereonreproduced in reproducing head 210 in the reverse direction from whichthe signal was recorded, which results in the signals being again in itsnormal sequence. The output of reproducing head 210 is supplied tofilter 31, which filtering network has substantially the same electricalcharacteristics as filter 31. Filter 31', therefore, additionallyfilters the signal and removes the phase distortion introduced into thesignal by filter 31. The output signal from filter 31 is supplied tosignal processing means 25 and thence to recording means 26 and/ ordisplay means 27.

It will thus be seen that I have provided method and apparatus foroperating on a seismic detector signal in such a manner as tosubstantially remove undesired phase distortion introduced either intothe seismic waves in traveling through the earth or into theseismicdetector signal by components of the recording and/ or analyzingsystem. In the case of phase distortion in the seismic waves, the phasedistortion is removed by reversing the time sequence of the seismicdetector signal and playing the reversed signal through an electricalnetwork having an electrical transfer function analogous to the acoustictransfer function of the portion of the earth in which the phasedistortion occurred. In the case of phase distortion produced bycomponents of the recording system, the distortion is removed byreversing the time sequence of the seismic detector signal and playingthe reversed signal through either the distorting device itself or asubstantial duplicate thereof.

It will be obvious to those skilled in the art that variousmodifications of the illustrated embodiments may be made withoutdeparting from the spirit of the invention or the scope of the appendedclaims. In particular, it Will be apparent that a seismic detectorsignal may be repeatedly passed through an electrical network or device,such as a geophone, an amplifier or a filter, an even number of timeswith the direction of the signal reversed for half the number of suchpassages to substantially eliminate phase distortion introduced into thesignal by the electrical device. Such a method of operation isparticularly advantageous in the case of filtering a seismic signal,where, by passing the signal repeatedly through a network in the mannertaught by this invention, the slope of the sides of the over-allfrequency response curve may be made as steep as possible withoutintroducing any substantial phase distortion into the final outputsignal.

. Iclaim:

Apparatus for recording seismic vibrations comprising a seismic detectoroperable to translate vibrations into a seismic electric signal, amagnetic recorder, a field filter having its input connected to saidseismic detector and its output connected directly to said magneticrecorder, said field filter being operable to restrict the frequencyband of the seismic signal which is actually recorded and simultaneouslyto distort the seismic signal, means for reproducing the recorded signalin the laboratory in reverse time realitonship from that in which it wasrecorded, a second and a third filter, said second filter beingconnected directly between said reproducing means and said third filterand having distortion and frequency passband characteristicssubstantially the same as said field filter to remove the distortionintroduced thereby, said third filter having a different frequencycharacteristic than said second filter and being operable to distort thesignal, a second magnetic recorder connected to the output of said thirdfilter, means for reproducing the signal recorder by said secondrecorder in reverse time relationship from that in which it was recordedto return the time sequence to that of the original seismic signal, afourth filter, and an optical recorder, said fourth filter having itsinput connected directly to said lastnamed reproducing means and itsoutput connected to said optical recorder and being the only filterbetween said last-named reproducing means and said optical recorder andhaving distortion and frequency characteristics substantially the sameas said third filter to remove the distortion introduced thereby.

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